It is often desirable to process fluids continuously through apparatus at elevated pressures. When such operations are carried out it is necessary to insure that the internal flow paths of the liquid being processed do not become short-circuited in such a way as to allow the feed stream to become mixed with the stream which has already been processed. Naturally, it is desirable to achieve these ends with the least cumbersome apparatus possible Generally, since the apparatus will be used in laboratory benchtop operations, it should be as compact and as light in weight as possible. In addition, it is desirable that the apparatus be easy to use, and disposable.
Ultrafiltration and microfiltration are chemical processing operations that are useful as practical operating methods for a large number of applications. The single most important step in achieving sustained practical filtration rates was the development of a family of anisotropic polymeric ultrafiltration and microfiltration membranes of various molecular weight cutoffs (MWCO) having effective pore sizes of from about 10.ANG. to the micron range. These membranes exhibit very high flow rates at normal operating pressures, and good resistance to plugging. The latter phenomenon is believed to be largely assignable to a very thin barrier layer on the upstream side of the membrane which allows the membrane to perform as a surface-type rather than a depth-type filter. Such ultrafiltration and microfiltration membranes as described herein are commercially available under the trademark DIAFLO.RTM. from W. R. Grace & Co.-Conn.
In order to maintain a continuous flow of liquid at the desired high flow rates using these membranes, a widely accepted technique called "cross-flow" (or tangential-flow) filtration may be used in order to process a large volume of liquid in a short time. (Since cross-flow filtration is useful with microfiltration as well as ultrafiltration membranes, the word "filtration" will be used hereinafter to refer to both "microfiltration" and "ultrafiltration".) Rather than employing "well-stirred batch cells," i.e., cells wherein the solution in the cell to be filtered is constantly stirred by an agitating device riding just above the membrane surface at the bottom of the cell, in cross-flow filtration fluid flows across, or tangential to, the filtration membrane surface. High process flux is thus achieved because only the thin membrane surface acts as the sieving barrier, as opposed to depth filtration, where the whole thickness of the membrane is used. Cross-flow filtration also avoids the well-known problem of "concentration polarization," wherein solute rejected by the filtration membrane accumulates on the membrane surface to form a gel-like film which prevents further filtration.
A typical apparatus for carrying out cross-flow filtration is described in U.S. Pat. No. 3,556,302, the disclosure of which is herein incorporated by reference. Such a device comprises two deflection resistant plates and a flow-distributing means between said plates which flow-distributing means is maintained in intimate contact with the face of a filter by a hydraulic pressure difference maintained across the distributor plate. Such pressures are usually on the order of from about 30 psi. The hydraulic pressure difference ensures that the fluid flow channel formed by contact of the flow-distributing means and the membrane surface is not short-circuited. This results in a higher quality of filtration, but such units are very bulky and made of interlocking parts (often machined out of 316 stainless steel) which must be assembled and disassembled each time a new membrane is used; thus the unit is prone to leakage if not assembled properly. Integral filtration units reduce the leakage problem, but suffer the same problem of bulkiness.
Another problem associated with filtration units of this type is the physical deformation the membrane is subjected to from pulses from a fluid delivery device. A peristaltic pump is commonly used to deliver fluid to the unit; these pumps do not provide pulse-free fluid delivery. The fluid enters the device and the fluid pulses are absorbed by the deflection-resistant plates; however, this means that the more compliant distributor plate must "give", and it presses down on the compliant membrane with each pulsation of the fluid stream. This causes the membrane to physically "wear" faster than desired. Although some membrane materials may withstand this abuse, others cannot. The flow rate of the pump may be reduced to avoid the above problems, but at the expense of dimished throughput.
Thus, it is an object of this invention to provide a lightweight simple cross-flow filtration device which allows the highest permissible fluid flowrates, is advantageously disposable, leak-free, and which preserves the physical integrity of filter membranes by reducing fluid pulsation.
Further objects of the invention are to describe a method for concentrating fluid utilizing the device of the invention, and a fluid concentration system.
Other objects of the invention will be apparent to those skilled in the art upon reading this specification.